function preload() {
    var img1 = new Image();
    img1.src = "images/nav1_on.png";
    var img2 = new Image();
    img2.src = "images/nav2_on.png";
    var img3 = new Image();
    img3.src = "images/nav3_on.png";
    var img4 = new Image();
    img4.src = "images/project_on.png";
    var img5 = new Image();
    img5.src = "images/software_on.png";
    var img6 = new Image();
    img6.src = "images/hardware_on.png";
    var img7 = new Image();
    img7.src = "images/study_on.png";
}